I haven't levelled my DK yet (and I don't think I plan to for some time), but I did level my pally, and went straight from Hellfire, to Nagrand (with a few dungeons along the way), and was 68 in no time.

I made it a point to go to Nagrand because I never did on my rogue, and missed out on a sizeable chunk of reputation that I wanted later anyway.

Why stop at 68? I lvled my DK (now the MT for my guild in Nax and Uld) up to 75 getting Outland Loremaster. I did this for two reasons. First I wanted the achievement and most important. It was faster lvling in Outlands.

I say its faster because even though most the quest are green, I had a epic flying mount to zip from quest to quest. When I hit 68 I traveled to one of the starter zones in Northrend long enough to hit 70 and gain those nice gear upgrades from the starter quest. I then went back to Outlands and grabed my Epic Flyer and started clearing zones. With the epic flyer and Northrend starter gear I could easily solo an entire zone in a few hours and gain a lvl. I left Outlands at 75 and hit 78 by the time I got to Dragonblight. So I only had 2 zones I couldnt use my epic flyer in Northrend to get to 80. From there I had like 5 zones worth of quest to simply gain gold instead of xp.

The only negative to this method is you are higher lvl the the zone dungeons and cant que up for them, but I usually just grouped with guildies.

Ive used this method for 3 characters now. My DK, Druid, and Paly... all three lvled extreamly fast compared to other guildies who went straight for Northrend. Epic Flight FTW imo.

Could do it that way if you want...but I'm not really a big fan of Outland. Aside from the intitial quests in Hellfire (the ones that give you that OH SHIT HURRY UP OR THE LEGION BREAKS THROUGH) feeling, I consider most of the ten levels I spend in Outland to be agonizing, so I get out of there as fast as I can.

Protips:

1) Go blood. The amount of survivability you'll have is absurd, literally nothing can kill you up to the harder quests in Storm Peaks and Icecrown.

2) Do Outland dungeons. The quests at Hellfire and Terokkar are OK, but they don't offer much lorewise and are not very exciting either.

3) When you hit 68, GTFO Outland. Northrend is TONS more fun, just as easy, has less running involved, has rewards are superb, some blues last you to 80, XP and G is also great and the dungeons are a blast. Do NOT stay in Outland, it will kill the whole leveling experience.

"...the rewards are superb..."
Sorry bout that.

Also, get the Hellreaver from hellfire ramparts. It looks insanely cool and is a great weapon overall.

http://www.wowhead.com/?item=24044

I also never stopped in Zangarmarsh. By the time you've tied Hellfire, you can pretty easily handle Terrokar, although I agree that Nagrand is money.

There IS however one quest you must definitely hit in Zangarmarsh;
http://www.wowhead.com/?quest=10096
which can give you, as a reward, this:
http://www.wowhead.com/?item=27715

It looks ugly as sin, but check out those stats... and three (3!) sockets. Hit the AH and pop a bunch of Wrath jewels in there, open up the display options and hide your helm, and you're ready for a rip-roarin ride. It have everything a DK wants, Str, Stam and crit... and no spellpower, like at least half of the @#@! plate in Outland.

get [Blade of Misfortune] ASAP!
http://www.wowhead.com/?item=31134